I have recently come up with a way to help bi folks explain how being bi works to people who like to say things like "you'll just end up gay or straight if you're married" or "you end up picking a side in the end".
Use what I'll call the baseball metaphor. Being bi is like having 2(+) favorite baseball teams. If you're dating someone it's like having a favorite player on one of the teams. Dating them doesn't diminish your like for either team or mean that their team is now your favorite. You may later have a new favorite player on a different team, but you still like the different teams just as much.
For straight/gay/lesbian people it would be like having one favorite team and their favorite player is always from that team even if the favorite player changes over time.
I will always see bisexuality as picking a favorite person and not picking a gender you prefer. Frankly, any relationship is just picking a favorite person. I hope this helps because it was the only way I could get my parents to understand.
P.S. I know next to nothing about baseball and nor do I speak for everyone in the LGBTQ+ community, I just wanted to help others explain something that is often messed up
